Pages

Thursday, July 26, 2018

Getting Ahead

I was so excited to get past this hurdle - heads are finally (almost) in shape. I figured the best way to CC my heads was to install the valves and valve springs - turns out I was wrong*, but that's how I did it to start. Got them in pretty easy, once I figured out the tool:

I'm really, really opposed to buying single-purpose tools. However, there's really no way around this one - it was either this or some kind of press with a few hokey jigs, and I didn't have space for that nonsense.

Once I got the valves installed, I started prepping for the CC measurement process:

Ok, the first step is always to get the heads level. I figured the easiest way to do that was to set up the heads, then add shims until they were level. This turned out to be a huge project, because even with the heads just about perfect, air bubbles would still show up *not* directly under the hole in the plexiglas. So I'd have to start tipping the head, and by the time I got the bubbles where I needed them, the shims would have walked out of position and now the head isn't level anymore, and the bubbles zip off to some corner of the chamber. I spent more time re-stacking shims than I did on any other part of this project!

I failed to take pictures of the entire CC process - call me impatient, I just didn't want to stop what I was doing and get my camera out. I went through the process at least 3 times for each chamber, then averaged my results - which wasn't hard, considering my syringe only had 1cc graduations. I was still able to guesstimate quarter-cc accuracy though.

Final volumes were 56cc, 56cc, 56cc, and 55cc. I'm still not sure about the last one - but it was consistent, all three measurements showed 55cc on the dot. That was easy enough to fix with a rotary tool and a cutter, followed by a sanding drum to smooth away chatter marks. Now I've got about a quarter cc spread between all four chambers, which is definitely good enough for a VW.

I cleaned the heads and valves really well to make sure no shavings or abrasives remained, and then put them away for now. I need to measure the valve spring forces, but after that I can reassemble the heads and bag them for now.

Aside from matching all the chamber volumes, CCing the heads gives me the last unknown for my compression ratio equation. I've got 94mm cylinders with a 78mm stroke; that's good for 541.2918 cubic centimeters. I've got 56 cc's in the heads, and I want a compression ratio (CR) of 8:1. CR is calculated by adding the volumes of the cylinder, head, and deck height, and then dividing by the sum of the head and deck height. When I go through all that, I get a necessary deck height of 3.06mm, or about 0.120". Back in an earlier post, I made mention that with the cylinder shims that were on the engine, the deck height was 0.114"..... which is pretty close to what I want. I can add .01" by going with the next size up shim; if I felt really crazy, I could chuck up the cylinders and turn them down .004" to get exactly what I'm after. However, I'm so close to final assembly of all of these components, I don't want to risk damaging something - and I think being off a tenth on my CR is probably ok; the math I'm finding looks like I'd be splitting hairs over less than half a percent of my power.

*The easiest way to do it is smear just a little grease on the valves and on the valve seats, then push the valves in and give them a little turn to hold them in place. No leaks this way.